No, it's not, but if you know the requester only wants you to do it once and you try to scam them out of bonus payments after you find out about it, then it is.
I catch HITs I have done before all of the time and if they state that they only want it done once, I throw it back or let it expire. Simple. I don't just do it because I caught it and "oh well, the requester should have known better LOL" These requesters are often times new to this platform and do not know how to screen or qual people out of tasks. They are trying to do research that could be ruined by people responding to it multiple times. If you want to do scam a requester and possibly ruing their research that's up to you, I guess, but don't come on to a forum and basically announce that you are going to do it. Believe it or not, requesters check forums like this more often than you think. If they see people posting "Hey, do you think it's alright if I scam this dude because he accidentally paid me twice?" They probably aren't going to be too excited about putting their research up on this platform.
So yeah, eyes rolled.